Ion implantation is a standard technique for introducing conductivity-altering impurities into substrates. A precise doping profile in a substrate and associated thin film structure is critical for proper device performance. Generally, a desired impurity material (i.e. dopant) is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the bulk of the substrate material and are embedded into the crystalline lattice of the substrate material to form a region of desired conductivity. The energetic ions bombardment damages the single-crystal structure of the substrate material after the ion implantation. A high-temperature (more than 1000 degrees Celsius) annealing process is required to repair the damage and to activate the dopant.
Ion implantation can independently control both dopant concentration and junction depth. Dopant concentration can be controlled by the combination of ion beam current and implantation time, and junction depth can be controlled by the ion energy. Ion implantation is a high vacuum process, and a thick layer of photoresist can block the energetic dopants ions. Ion implantation can use photoresist as the patterning mask and does not need to grow and etch silicon dioxide to form the hard mask as the diffusion doping process does.
The mass analyzer of an implanter selects exactly the ion species needed for implantation and generates a pure ion beam; thus ion implantation has less possibility for contamination, the ion implantation process always operates in a high vacuum, an inherently clean environment, and is an anisotropic process. Dopant ions are implanted into the silicon mainly in the vertical direction, and the doped region closely reflects the area defined by photoresist mask.